The invention relates to a microwave oven comprising an oven cavity in which an article to be heated is placed, a microwave generator for radiating microwave energy into said cavity for heating said article, fan means for producing an air flow in said cavity, humidity sensor means for sensing a change in humidity of the air in said cavity and control means for controlling the supply of energy to the article in dependence on the sensed humidity.
In particular, when the oven is operating in the defrost mode, the heating process can be rapid, causing the temperature in certain portions to rise to an unacceptably high level if the process is not interrupted in proper time. To avoid overheating it is then common practice to use a low power level so that on the one hand the process proceeds very slowly and is easier to check visually and on the other hand that due to the slow process more time is available for temperature equalization so that heat from overheated parts is led to colder parts.
It is difficult to control the defrosting process automatically because it is very difficult to detect the instantaneous defrosting condition. A known method is based upon the principle of sensing the variations in the dielectric properties taking place at the ice-water phase transition by means of microwave energy--if desired of another frequency than that of the energy supply. However, this method is inaccurate and often results in disturbances, on the one hand due to the fact that the geometry of the food varies and on the other hand due to the ionic conductivity caused by the salt content. Another known method is based upon the principle of sensing the weight of the article as a function of the energy supply. This method has also disadvantages. It does not enable the energy required for defrosing to be calculated in a sufficiently exact manner, because this energy, for equal weight, also depends to a substantial degree on the type of articles, its water content and its temperature on initiating defrosting. In addition, with this known method, account must also be taken of the weight of the plate or tray on which the article is disposed in the oven chamber. Because of these facts, the article is frequently overheated. If the defrosting mode is controlled by means of weight detection it is known to control the following cooking mode by means of humidity detection. In this method humidity detection is also operable during the defrosting mode if errors might occur in abnormal circumstances, e.g. when the article has been abnormally defrosted.